density parameter

The density parameter (Ω) is the ratio of the actual mean density
of mass-energy in the universe (ρ) to the critical
density (ρc). A value of Ω greater than 1 implies
that the universe will eventually collapse and corresponds to the case of
a closed universe (i.e. one with positive curvature like that of a sphere); a value less than one suggests eternal expansion
and corresponds to the case of an open universe (i.e. with negative curvature
like that of the surface of saddle. The actual value remains to be determined
but is thought to be close to unity (equivalent to a flat universe). See
also cosmological model.